0.4 to 0.5

When used with the UTF-8 encoding performance improvements introduced
in version 0.11.1.12 of the text package, this release improves
aeson's JSON encoding performance by 33% relative to aeson 0.4.

As part of achieving this improvement, an API change was necessary.
The fromValue function in the Data.Aeson.Encode module now uses
the text package's Builder type instead of the blaze-builder
package's Builder type.

0.3 to 0.4

Ease of use

The new decode
function
complements the longstanding encode function, and makes the API
simpler.

The modern GHC-based generic mechanism is fast and terse: in fact, its
performance is generally comparable in performance to hand-written and
TH-derived ToJSON and FromJSON instances. To see how to use GHC
generics, refer to
examples/Generic.hs.

The SYB-based generics support lives in
Data.Aeson.Generic,
and is provided mainly for users of GHC older than 7.2. SYB is far
slower (by about 10x) than the more modern generic mechanism. To see
how to use SYB generics, refer to
examples/GenericSYB.hs.

Improved performance

We switched the intermediate representation of JSON objects from
Data.Map to
Data.HashMap,
which has improved type conversion performance.

Instances of ToJSON and FromJSON for tuples are between 45% and
70% faster than in 0.3.

Evaluation control

This version of aeson makes explicit the decoupling between
identifying an element of a JSON document and converting it to
Haskell. See the
Data.Aeson.Parser
documentation for details.

The normal aeson decode function performs identification strictly,
but defers conversion until needed. This can result in improved
performance (e.g. if the results of some conversions are never
needed), but at a cost in increased memory consumption.

The new decode' function performs identification and conversion
immediately. This incurs an up-front cost in CPU cycles, but reduces
reduce memory consumption.